A man is being questioned by police investigating an alleged assault on Labour MP for Barrow and Furness John Woodcock.
Mr Woodcock said he had a bag shoved in his face and was threatened after he challenged two men over anti-social behaviour on a train.
The incident happened at around 10.30pm last Thursday after the shadow transport minister boarded the train at Lancaster, heading to Barrow, British Transport Police said.
Earlier on Monday, the force released a CCTV picture of a man they wanted to speak to in connection with the incident and appealed for witnesses.
The appeal has since been cancelled after a man handed himself in.
A spokesman said: “Following an appeal and photograph issued earlier today, a man has handed himself in to police and is currently being questioned by officers.
“The appeal related to an incident on board a train on Thursday February 23 in which a 33-year-old man was assaulted after challenging a group of male passengers about their behaviour.”